Re: [RFC PATCH RESEND 0/5] amd_iommu: support up to 2048 MSI vectors per IRT

[Sairaj Kodilkar <sarunkod@amd.com>]

Hello all,

Gentle ping,

On 11/18/2025 3:45 PM, Sairaj Kodilkar wrote:
> Resending this series with KVM and IOMMU maintainers in CC.
>
> AMD IOMMU can route upto 2048 MSI vectors through a single
> Interrupt Remapping Table (IRT) entry. This series brings the same
> capability to the emulated AMD IOMMU in QEMU.
>
> Highlights
> ----------
> * Sets bits [9:8] in Extended-Feature-Register-2 to advertise 2K MSI
>    support to the guest.
> * Uses bits [10:0] of the MSI data to select the IRTE when the guest
>    programs MSIs in logical-destination mode.
> * Introduces a new IOMMU device property:
>          -device amd-iommu,...,numint2k=on
>
>    The feature is **opt-in**; guests keep the 512-MSI behaviour unless
>    `numint2k=on` is supplied.
>
> Passthrough devices
> -------------------
> When a PCI function is passed through via iommufd the code checks the
> host’s vendor capabilities.  If the host IOMMU has not enabled
> 2K-MSI support (bits [44:43] set in the control register) the guest
> feature is disabled even if `numint2k=on` was requested.
>
> The detection logic relies on the iommufd interface; with the legacy
> VFIO container the guest always falls back to 512 MSIs.
>
> Example
> -------
> qemu-system-x86_64 \
> -enable-kvm -m 10G -smp cpus=8 \
> -kernel /boot/vmlinuz \
> -initrd /boot/initrd.img \
> -append "console=ttyS0 earlyprintk=serial root=<DEVICE>"
> -device amd-iommu,dma-remap=on,numint2k=on \
> -object iommufd,id=iommufd0 \
> -device vfio-pci,host=<DEVID>,iommufd=iommufd0 \
> -global kvm-pit.lost_tick_policy=discard \
> -cpu host \
> -machine q35,kernel_irqchip=split \
> -nographic \
> -smbios type=0,version=2.8 \
> -blockdev node-name=drive0,driver=qcow2,file.driver=file,file.filename=<IMAGE> \
> -device virtio-blk-pci,drive=drive0
>
> Limitations
> -----------
> This approach works well for features queried after IOMMUFD
> initialization but cannot handle features needed during early QEMU
> setup, before IOMMUFD is available.
>
> A key example is EFR2[HTRangeIgnore]. When this bit is set, the physical
> IOMMU treats HyperTransport (HT) address ranges as regular memory
> accesses rather than reserved regions. This has important implications
> for memory layout:
>
> * Without HTRangeIgnore: QEMU must relocate RAM above 4G to above 1T on
>    AMD platforms to avoid HT conflicts
> * With HTRangeIgnore: QEMU can safely place RAM immediately above 4G,
>    improving memory utilization
>
> Since RAM layout must be determined before IOMMUFD initialization, QEMU
> cannot use hwinfo to query EFR2[HTRangeIgnore] feature bit.
>
> Another limitation with using the control register is that, if BIOS enables
> particular feature (e.g. ControlRegister[GCR3TRPMode) without kernel support
> QEMU incorrectly assumes that host kernel supports that feature potentially
> causing guest failure.
>
> Alternative considered
> ----------------------
> We also explored alternate approach which uses KVM capability
> "KVM_CAP_AMD_NUM_INT_2K_SUP", which user can query to know if host
> kernel supports 2K MSIs. Similarly, this enables qemu to detect the
> presence of EFR2[HTRangeIgnore] during RAM initialization.
>
> Although current implementation allows 2K MSI support only with
> iommufd, it keeps the logic inside the vfio/iommufd and avoids
> modifying KVM ABI. I am happy to discuss advantages and drawbacks of
> both approaches.
>
> ------------------------------------------------------------------------
>
> The patches are based on top of bc831f37398b (qemu master). Additionally
> it requires linux kernel with patches[1] which expose control register
> via IOMMU_GET_HW_INFO ioctl.
>
> [1] https://lore.kernel.org/linux-iommu/20251029095846.4486-1-sarunkod@amd.com/
>
> ------------------------------------------------------------------------
>
> Sairaj Kodilkar (3):
>    vfio/iommufd: Add amd specific hardware info struct to vendor
>      capability
>    amd_iommu: Add support for extended feature register 2
>    amd_iommu: Add support for upto 2048 interrupts per IRT
>
> Suravee Suthikulpanit (2):
>    [DO NOT MERGE] linux-headers: Introduce struct iommu_hw_info_amd
>    amd-iommu: Add support for set/unset IOMMU for VFIO PCI devices
>
>   hw/i386/acpi-build.c               |   4 +-
>   hw/i386/amd_iommu-stub.c           |   5 +
>   hw/i386/amd_iommu.c                | 163 +++++++++++++++++++++++++++--
>   hw/i386/amd_iommu.h                |  24 +++++
>   include/system/host_iommu_device.h |   1 +
>   linux-headers/linux/iommufd.h      |  20 ++++
>   6 files changed, 207 insertions(+), 10 deletions(-)
>